1. Field of the Invention
The present invention relates to an inductor, and particularly to an inductor which works practically and effectively in a high-frequency band.
2. Description of the Related Art
It has been becoming more and more important to reduce the noise of electronic apparatus as the electronic apparatus are required to be downsized and achieve higher performance. In order to reduce the noise, various types of inductors have been used. For example, for heavy current applications and in a relatively low-frequency band, a ferrous dust magnetic core and an amorphous magnetic core both having a high saturation magnetic flux density have been used, and they are mainly shaped a toroidal. On the other hand, for use in a relatively high-frequency band, an Nixe2x80x94Zn ferrite having a high resistivity (102 to 105 xcexa9m) has been used.
In recent years, there has been a growing demand for high-frequency inductors since the electronics apparatus are increasingly required to have higher performances at higher frequencies. The aforementioned Nixe2x80x94Zn ferrite is preferred also because a wire can be wound directly on the magnetic core owing to its high resistivity. However, since the Nixe2x80x94Zn ferrite has a low saturation magnetic flux density, it is not often used in a closed magnetic path, but is often used as a drum-shaped or a rod-shaped magnetic core which is an open magnetic.
As described above, the Nixe2x80x94Zn ferrite has been used in an inductor for a high-frequency application. However, the Nixe2x80x94Zn ferrite requires a special purpose manufacturing process because the Nixe2x80x94Zn ferrite contains Ni in its raw material thereby raising the problem with manufacturing cost and technology. On the other hand, an Mnxe2x80x94Zn ferrite which is inexpensive and shows superior characteristics generally has a low resistivity, ranging from 0.1 to 1 xcexa9m. As a result, an eddy current loss starts to increase even at a low frequency, and therefore, the Mnxe2x80x94Zn ferrite can be used only up to a few hundred kHz. In a frequency band exceeding a few hundred kHz, the Mnxe2x80x94Zn ferrite has magnetic permeability (initial permeability) remarkably decreased and totally loses its soft magnetic characteristic. The Mnxe2x80x94Zn ferrite, which has a low resistivity as mentioned above, requires an insulation covering or coating to prevent insulation failure which prohibits a wire from being wound directly on the core, resulting in increased cost, thus substantially limiting its applications.
In general, an equivalent circuit of an inductor is simply formed by a series equivalent circuit which is composed of a resistance component R and an inductive reactance L. More specifically, as shown in FIG. 5, it is formed by a series-parallel circuit which is composed of a series combination of the inductive reactance L and its resistance component R1 and another series combination of a capacitive reactance C and its resistance component R2. Here, the capacitive reactance C consists of a stray capacitance produced between the wires and another stray capacitance produced between the core and the winding. The resistance component R1 of the inductive reactance L consists of a resistance of a copper loss due to a wire resistance and another resistance due to a magnetic loss of the magnetic core. On the other hand, the resistance component R2 of the capacitive reactance C consists of a loss (the loss depends on a dielectric loss as described later) caused by an electric coupling between the core and the winding. The equivalent circuit thus formed causes an LC resonance in a frequency characteristic of the inductor, showing the hill-like impedance characteristic curve.
Q factor is a well-known indicator or sharpness of the LC resonance of the inductor. A large Q factor causes a sharp resonance and a smaller Q factor causes a less sharp resonance. The Q factor of the inductor is approximately determined by the environment of an electronic circuit. In recent years, since electric apparatuses have been required to be adapted for higher frequency and to be digitalized, inductors capable of reducing the high-frequency noise are becoming more and more important. In addition, parts provided with countermeasures against noise which efficiently absorb noise components without distorting the transmission signal wave are increasingly demanded.
When the resonance is caused with a sharp impedance of an inductor due to a large Q factor, the inductance changes sharply according to the resonance frequency, thereby causing noise and possibly distorting the transmission signal wave. Therefore, an inductor is demanded which does not produce the above mentioned resonance with a sharp impedance characteristic and which can be duly used in a high-frequency band.
As described above, the magnetic core made of a soft magnetic material such as an Mnxe2x80x94Zn ferrite is inexpensive and shows superior characteristics in a low frequency band, but since the Mnxe2x80x94Zn ferrite is very low in resistivity, its eddy current loss starts to increase even at a low frequency, and therefore the Mnxe2x80x94Zn ferrite can be used only up to a few hundred kHz. And the Mnxe2x80x94Zn ferrite requires an insulation covering or coating to prevent insulation failure caused by the low resistivity, which means a wire cannot be wound directly on the magnetic core, thus leading to increased cost. In order to solve the above conventional problems, the present inventors have disclosed in Japanese Patent Nos. 3108803 and 3108804 in which an Mnxe2x80x94Zn ferrite which has its resistivity remarkably increased by limiting Fe2O3 content to less than 50.0 mol %, and in addition, by allowing a suitable amount of TiO2 or SnO2 to be contained.
However, an inductor just using a ferrite with a high resistivity as a magnetic core cannot successfully reduce the noise without distorting the transmission signal wave. This is true of an Nixe2x80x94Zn ferrite. When the Nixe2x80x94Zn ferrite is used as a magnetic core, a resonance is caused in which the impedance characteristic, that is, a practical characteristic is sharp.
As described above, when the Nixe2x80x94Zn ferrite is used as a magnetic core of an open magnetic path, the Q factor is large, thereby making the impedance of the inductor sharp in resonance. The Q factor is inversely proportional to a loss component of the inductor part. On the other hand, the loss component of the magnetic core involves, as described above, the magnetic loss and the dielectric loss (the ratio of an imaginary part to a real part of a relative complex dielectric constant), and the winding loss component involves the wire resistance. Out of these components, the magnetic loss and the dielectric loss depending on the characteristics of their materials are small in the Nixe2x80x94Zn ferrite, and consequently the Q factor is large making the impedance of the inductor to easily resonate sharply. Therefore, an inductor is demanded which does not produce such a sharp resonance and, at the same time, which can be used in a high-frequency band.
The present invention has been made in view of the above described circumstances in the prior arts.
A first object of the present invention is to provide an inductor made of an inexpensive Mnxe2x80x94Zn ferrite which has its resistivity substantially increased thereby obtaining the same high-frequency characteristics as with the Nixe2x80x94Zn ferrite and the same time enabling a wire be wound directly on the magnetic core of the inductor. A second object of the present invention is to provide an inductor which can reduce its noise without adversely affecting the transmission signal wave form.
In order to achieve the above objects, according to a first aspect of the present invention, in an inductor, which comprises an open magnetic path formed by a soft magnetic material and a winding provided around the open magnetic path a relative complex dielectric constant of the soft magnetic material varies according to a frequency, and an imaginary part of the relative complex dielectric constant is greater than a real part thereof in a high frequency band equal to and higher 1 MHz. Consequently, the inductor of the present invention achieves a high-frequency characteristic equivalent to that by the conventional Nixe2x80x94Zn ferrite inductor, allows the winding to be provided directly on the core has an excellent impedance characteristic, and does not affect adversely the transmission signal wave form.
According to a second aspect of the present invention, in the inductor of the first aspect, the soft magnetic material has a resistivity of at least 150 xcexa9m and has a real part of the relative complex dielectric constant ranging between 1,000 and 20,000 including 1,000 and 20,000 at 1 kHz, and 50 or less at 1 MHz. Consequently, the inductor of the present invention can be duly used in a practical frequency band.
According to a third aspect of the present invention, in the inductor of the first aspect, the soft magnetic material has a basic component composition of an Mnxe2x80x94Zn ferrite comprising 44.0 to 50.0 mol % Fe2O3 (excluding 50.0 mol %), 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % at least one of TiO2 and SnO2, and the remainder consisting of MnO. Consequently, the inductor of the present invention can be duly used in a practical frequency band.
According to a fourth aspect of the present invention, in the inductor of the first aspect, the soft magnetic material has a basic component composition of an Mnxe2x80x94Zn ferrite comprising 44.0 to 50.0 mol % Fe2O3 (excluding 50.0 mol %), 4.0 to 26.5 mol % ZnO, 0.1 to 8.0 mol % at least one of TiO2 and SnO2, 0.1 to 16.0 mol % CuO, and the remainder consisting of MnO. Thus, the inductor of the present invention can be formed basically of the inexpensive Mnxe2x80x94Zn ferrite with a high resistivity and therefore can be low in cost and high in performance.